The goal of the proposed research is to understand the processes which control differentiation of myogenic cells at a molecular level. The method of approach will be to analyze various properties of a permanent rat myogenic cell line which exhibits a characteristic muscle development when cultured in vitro under appropriate conditions. By comparing the control of differentiation of this cell line and its conditional temperature sensitive variants we hope to be able to establish some cause-effect relationships in the differentiation of these cells. We will study what controls are exerted in these cells on the expression of myosin heavy chain (MHC) genes during differentiation. We will determine the transcription, processing and stability of MHC mRNA in the wild type and several mutant cell lines using a recently characterized recombinant plasmid which carries MHC sequences. Studies in the whole cells will be complemented by experiments designed to study transcription of MHC in vitro isolated nuclei.